When a Titanium Sheath Heater Is Installed Horizontally in a 25% Sodium Chloride Brine at 95°C, How Does the Crevice Gap Under a PTFE Support (0.1 mm vs. 0.5 mm) Change the Time to Crevice Corrosion of the Titanium?

A titanium sheath heater supported on PTFE brackets in 25% salt chloride brine at 95°C showing crevice corrosion at the support contact locations. The severity depends on the crevice gap. At a tight gap of 0.1 mm, brine is excluded and crevice corrosion is not initiated. There is a 0.5mm space where the brine sits and enters. Chlorides concentrate to 50,000 - 100,000 ppm Oxygen is depleted pH drops Crevice corrosion begins between 200 and 500 hours and proceeds at a rate of 0.5 to 1.5 mm each year.

Mechanism of Crevice Corrosion as a Function of Gap Size

Capillary forces in a small gap (<0.2 mm) impede electrolyte entry. Moderate gap (0.2-0.5 mm) electrolyte penetrates but does not circulate. Oxygen is used up, chlorides become concentrated and the crevice turns acidic. For titanium in hot brine, the critical gap for crevice initiation is in the range of 0.1-0.3 mm. If the distance is >0.5 mm, then circulation is sufficient to avoid severe concentration.

Quantitative Crevice Initiation Time vs. Gap Size

Grade 2 titanium has been tested in 25% NaCl at 95°C under a PTFE support with the following results. At a gap <0.1 mm the initiation time of a crevice is greater than 10,000 hours and no corrosion occurs. The initiation time is 1,000-3,000 hours at a gap of 0.1-0.2 mm, and some minor pitting may occur. The initiation time is 300-800 hours with a gap of 0.2-0.3 mm. Crevice corrosion is moderate. At a gap 0.3-0.5 mm start time is 150-400 hours and severe crevice corrosion. Initiation period, 500-1500 hours at a gap >0.5 mm. Attack, less severe (improved owing to circulation).

Crevice Gap Design Guide

Crevice Gap (mm) Risk of Crevice CorrosionBeginning Time (hour)Recommended Support Design <0.1 None >10,000 Ideal (press fit)
0.1-0.2 Low 1,000-3,000 Acceptable
0.2-0.3 Moderate 300-800 Avoid 0.3-0.5 High 150-400 Avoid >0.5 Low (circulation) 500-1,500 Acceptable with drainage
Creating an Informed Specification

Design PTFE supports with a tight fit (<0.1 mm gap) or with an open gap >0.5 mm to allow circulation for a titanium heater in 25% NaCl at 95°C. Avoid gaps of 0.2-0.5mm. The engineer controls the crevice geometry to prevent crevice corrosion.